The performance of golf equipment is continuously advancing due to the development of innovative clubs and club designs. While all clubs in a golfer's bag are important, both scratch and novice golfers rely on the performance and feel of iron-type golf clubs (“irons”) for many commonly encountered playing situations.
Irons are generally configured in a set that includes clubs of varying loft, with shaft lengths and club head weights selected to maintain an approximately constant “swing weight” so that the golfer perceives a common “feel” or “balance” in swinging both the low-lofted irons and high-lofted irons in a set. The size of an iron's “sweet spot” is generally related to the size (i.e., surface area) of the iron's strike face, and iron sets are available with oversize club heads to provide a large sweet spot that is desirable to many golfers.
Conventional “blade” type irons have been largely displaced (especially for novice golfers) by so-called “perimeter weighted” irons, which include “cavity-back” and “hollow” iron designs. Cavity-back irons have an open cavity directly behind the strike plate, which permits club head mass to be distributed about the perimeter of the strike plate. Such cavity-back irons tend to be more forgiving to off-center hits. Hollow irons have features similar to cavity-back irons, but the cavity is enclosed by a rear wall to form a hollow region behind the strike plate. Perimeter weighted, cavity-back, and hollow iron designs permit club designers to redistribute club head mass to achieve intended playing characteristics associated with, for example, placement of a center of gravity (“CG”) or a moment of inertia (“MOI”) of the golf club head.
In addition, even with perimeter weighting, significant portions of the club head mass, such as the mass associated with the hosel, topline, or strike plate, are unavailable for redistribution. For example, the strike plate must withstand repeated strikes both on the driving range and on the course, requiring significant strength for durability.
Golf club manufacturers are consistently attempting to design golf clubs that are easier to hit and offer golfers greater forgiveness, such as when the ball is not struck directly at a “sweet spot” or center face of the strike face. As those skilled in the art will appreciate, many golf club head designs have been developed and proposed for assisting golfers in learning and mastering the game of golf.
With regard to iron-type club heads, cavity-back club heads have been developed. Cavity-back golf clubs shift the weight of the club head toward the outer perimeter of the club head. By shifting the weight in this manner, the CG of the club head is pushed toward the sole of the club head, thereby providing a club head that promotes better performance. In addition, weight is shifted to the toe and heel of the club head, which helps to expand the sweet spot and minimize negative performance characteristics associated with off-center strikes of a golf ball.
Shifting weight to the sole of the club head lowers the CG of the club head resulting in a golf club that launches the ball more easily and with greater backspin. Golf club designers often focus on the vertical CG of the golf club relative to the ground when the golf club is soled and in a proper address position. This vertical CG measurement is often referred to as Zup or Z-up or CG Z-up. Decreasing Z-up is preferable to increasing Z-up. Golf club designers seek to achieve a low Z-up both for golf clubs designed for low handicap golfers and high handicap golfers. For example, a low Z-up helps to maintain similar launch angles, but increases ball speed and distance, for low handicap golfers or a low Z-up helps to launch the ball more easily in the air for high handicap golfers. Additionally, placing weight at the toe increases the MOI of the golf club resulting in a golf club that resists twisting and is thereby easier to hit straight even on mishits.
As club manufacturers have learned to assist golfers by shifting the CG toward the sole of the club head, a wide variety of designs have been developed. Unfortunately, many of these designs shift the center of gravity toward the sole and perimeter of the club head at the expense of the appearance of the club head. For example, one method of lowering the CG is to simply decrease the face height at the toe and make it closer in height to the face height at the heel of the club resulting in a very untraditional looking club. This is highly undesirable as golfers have become familiar with a certain traditional style of club head and alteration of that style often adversely affects their mental outlook when addressing a ball prior to strike the ball. As such, a need exists for an improved club head which achieves the goal of shifting the CG further toward the sole and perimeter of the club head without substantially altering the appearance of a traditional cavity-back club head.
Unfortunately, the acoustical properties of a golf club head may be negatively impacted by relocating mass and lowering Z-up on the golf club head. The acoustical properties of golf club heads (e.g., the sound the golf club head generates upon impact with a golf ball) affect the overall feel of the golf club by providing instant auditory feedback to the user of the golf club. For example, the auditory feedback can provide an indication as to how well the golf ball was struck by the club, thereby promoting user confidence.
The sound generated by a golf club is based on the rate, or frequency, at which the golf club head vibrates and the duration of the vibration upon impact with a golf ball. Generally, for iron-type golf clubs, a desired first mode frequency is generally around 3,000 Hz and preferably greater than 3,200 Hz. Additionally, the duration of the first mode frequency is important because a longer duration may feel like a golf ball was poorly struck, which results in less confidence for the golfer even when the golf ball was well struck. Generally, for iron-type golf club heads, a desired first mode frequency duration is generally less than 10 ms and preferably less than 7 ms. Some conventional golf club heads employ features designed to increase the vibrational frequency of the golf club head and decrease the frequency duration of the golf club head. However, such features may fail to increase the vibration frequency of the golf club heads to desirable levels (e.g., a desirable upward shift in the vibration frequency) and/or decrease the frequency duration to desirable level.
Additionally, the coefficient of restitution (“COR”) of a golf club head may be negatively impacted by relocating mass and lowering Z-up on the golf club head. The COR of a golf club head is a measurement of the energy loss or retention when the golf ball is impact by the golf club head. Generally, the higher the COR, the more efficient the transfer of energy from the golf club head to the golf ball and the longer the golf shot. For some conventional golf club heads, lowering the Z-up of the golf club head results in an undesirable lowering of the COR.
Conventional iron-type golf club heads may not achieve desired first and fourth mode frequencies and frequency durations and desired COR characteristics while providing the performance benefits afforded by a low Z-up. Accordingly, it would be desirable to provide a golf club head that lowers the Z-up while maintaining desirable vibration frequency and duration characteristics and a desirable COR.